1. Field of Invention
This invention relates to the field of tractor hitches. More specifically, the invention comprises a device which adjustably sets the lowest point of travel for a conventional three-point agricultural hitch. The device is useful for maintaining an agricultural implement at the desired lowest point of travel without putting pressure on the hydraulic pump commonly used to adjust the height of such hitches.
2. Description of Prior Art
Three-point tractor hitches have been in common use for several decades. U.S. Pat. No. 4,071,105 to Von Allworden (1978) is a typical example. Similar configurations are disclosed in U.S. Pat. No. 2,685,160 to Kuhary et.al. (1954), U.S. Pat. No. 2,828,968 to Engler (1958), and U.S. Pat. No. 2,462,588 to Wondra (1947). The basic three-point configuration has changed little since its inception.
Turning to FIG. 1, the prior art three-point hitch will be explained. Tractor 10 has a pair of rocker arms 12, which rotate on a common shaft as hydraulic pressure is applied. When rocker arms 12 rotate upward, they pull lift links 16 with them. Lift links 16, in turn, pull lift arms 14 up with them. Lift arms 14 are pivotally connected to the underside of tractor 10 (illustrated in other views).
Many different agricultural implements may be attached to the hitching system. In the particular example illustrated in FIG. 1, mower 22 is attached. Mower 22 has A-frame 20 rigidly attached to its forward portion. A-frame 20 has two lift pins 24 rigidly attached thereto. Lift arms 14 have holes sized to slip over and engage lift pins 24.
Top link 18 is pivotally attached to tractor 10 as shown. The opposite end of top link 18 is pivotally attached to the upper portion of A-frame 24. The purpose of top link 18 is to keep mower 22 roughly level as it is raised and lowered (via the well-known operation of a 4-bar mechanism). Those skilled in the art will easily realize how the illustrated device functions. Rocker arms 12 are forcibly rotated by hydraulic power. If they are raised, then mower 22 is raised. Likewise, when they drop, mower 22 drops. Top link 18 is typically just a steel bar with an adjustment mechanism used to vary its length. The particular top link 18 illustrated in FIG. 1 is a new version, incorporating features of the present invention. However, it also provides the normal functions of a conventional top link.
Operator controls (not shown) are provided to allow the driver to adjust the height of the implement. It is very important to maintain a fixed height off the ground during many agricultural operations. The importance of this is obvious for mowing. It is also very important if the towed implement is a spraying rig--such as the type used for fertilizer and pesticides. The prior art operator controls often have an adjustable lower stop. The operator can set this stop to provide the correct height for the activity he is conducting. The height is hopefully maintained by hydraulic pressure holding rocker arms 12 in the set position. One must realize, however, that most agricultural implements are quite heavy.
In actual practice, this method of height maintenance has a serious shortcoming. The hydraulic pressure provided to position rocker arms 12 is supplied by a pump powered off the tractor's engine. The pressure is regulated by one or more control valves--usually of the proportional type. As the control valves wear with age, they often leak. This fact means that the height adjustment will creep downward. Over twenty or thirty minutes of operation, the operator will look back and realize that the implement has sunk six inches lower. He must then raise and lower the implement to reset the system. On older systems, he may have to reset the height every two or three minutes.
Even with new pumps and valves, height variations occur. On a cold morning, the hydraulic fluid circulating in the system starts with a much higher viscosity. As the fluid warms, its viscosity diminishes. Thus, an initial height adjustment will tend to sink as the system comes up to normal operating temperature. Significant differences in ambient temperature can cause the same phenomenon. A height setting made on a cold day will not bring the implement to the same position on a hot day.
The known devices for setting the height of an agricultural implement are therefore limited in that they:
1. Do not maintain a fixed position above the ground, instead letting the implement creep downward; PA1 2. Do not maintain a fixed position as the hydraulic system heats up under cold conditions; and PA1 3. Do not determine the same position under varying ambient conditions. PA1 1. To maintain the implement in a fixed position above the ground; PA1 2. To maintain said fixed position despite varying viscosity of the hydraulic fluid in the system; and PA1 3. To maintain said fixed position despite varying ambient conditions.